Any references to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.
Significant scientific effort has been invested in the realization of imaging and materials analysis systems over the past two decades. One outcome of this effort is that THz time-domain spectroscopy (TDS) has established itself as a significant tool for coherently probing solid-state, liquid, and gaseous systems at THz frequencies. Key to the success of THz TDS is its capability of measuring complex refractive indices of samples over bandwidths as large as 100 THz, due to its intrinsic ability to resolve the electric field amplitude of broadband THz pulses coherently and with subpicosecond resolution, as well as its insensitivity to thermal background radiation. However, THz TDS systems in general have signal-to-noise ratios (SNRs) that are practically useful only below ˜3 THz. Furthermore, their spectral resolution is typically limited to no better than ˜5 GHz (worse in high-bandwidth systems), and they are restricted to low THz powers on the order of 10-100 μW for commonly used optically-pumped photoconductive emitters.
Moreover, spectroscopic data acquisition is slow and the technique relies on bulky and expensive ultrafast laser sources for the generation and coherent detection of THz radiation.
Recently the THz quantum cascade laser (QCL) has emerged as the established laboratory source of high-power radiation in the frequency range ˜1-5 THz. THz QCLs have been shown to exhibit remarkable spectral purity with quantum-limited linewidths, making them ideally-suited to coherent THz systems. Nevertheless, owing to the challenges of coherently detecting the emission from such sources, most system developments have focussed on incoherent approaches to imaging and materials analysis. Coherent detection schemes have, however, permitted the phase and/or frequency of the THz field to be resolved. By exploiting the heterodyne mixing between a free-running QCL and a local oscillator derived from a gas laser, high-resolution frequency-resolved gas spectroscopy has been reported. Phase-sensitive detection using a heterodyne approach has also enabled coherent inverse synthetic aperture radar imaging. However, heterodyne systems generally suffer from the disadvantage that they are complex and bulky
It is an object of the invention to provide a laser based imaging or remote materials sensing system that is an improvement, or at least a useful alternative, to the aforementioned systems of the prior art.